1. Field of the Invention
The present invention relates in general to an extendable radio antenna for cordless telephones and for portable cellular telephones and, more particularly, to a structural improvement in such an antenna for making the antenna in the retracted state be operated as the equivalent of a half-wave dipole antenna and for compensating for deficient capacity component of a helical antenna element of the antenna in the retracted state and for improving the efficiency of whip antenna element as well as the efficiency of helical antenna element, and improving the mechanical reliability of the antenna.
2. Description of the Prior Art
In prior art extendable antennas for domestic cordless telephones and for portable cellular telephones, a quarter-wave whip antenna element and a quarter-wave helical antenna element are integrated into an extendable antenna. When the integrated extendable antenna is in the retracted state during the carrying or keeping of the telephone, the radio frequency signals are received by the helical antenna element exclusively. However when the extendable antenna is in the extended state while talking over the telephone, only the whip antenna element is in the signal transmission and signal reception mode.
With reference to FIGS. 1A and 1B, there is shown the prior art extendable antenna in the fully extended state and in the fully retracted state respectively.
In the typical extendable antennas for cordless telephones, the whip antenna element and the helical antenna element are separated from each other at an interval such that the whip antenna element is exclusively operated in the extended state of the antenna while the helical antenna element is exclusively operated in the retracted state of the antenna. With the structural simplicity of the above extendable antennas, the antennas have been wide used in recent years.
However, as the above antenna is operated only by the helical antenna element during the retracted state of the antenna, the antenna has a problem of narrow bandwidth and low radiation efficiency. Another problem of the above antenna is resided in that the mechanical coupling portion between the whip antenna element and the helical antenna element is such frail that the coupling portion is apt to be broken.